Removing undesired potential from the blocking electrode in a photoelectrophoretic imaging system

ABSTRACT

Build-up of undesired potential on the blocking electrode in a photoelectrophoretic imaging system during successive imaging operations is prevented by one of several methods. Typically, potential of a sign opposite to that built up may be applied, or the blocking electrode surface may be discharged between imaging cycles.

United States Patent Ciccarelli [54] REMOVING UNDESIRED POTENTIAL FROM THE BLOCKING ELECTRODE IN A PHOTOELECTROPHORETIC IMAGING SYSTEM abandoned.

[52] US. Cl. ..204/l81, 96/1 R, 96/1.2, 117/37 LE [51] Int. Cl. ..G03g 13/00 [58] Field of Search ..96/l, 1 C, 1 A; 250/495; 204/181 PE 51 July 11,1972

[56] References Cited UNITED STATES PATENTS 3,384,565 5/1968 Gulagin et a1 ..96/1.2 X

3,013,878 12/1961 Dessauer ..96/l 2,945,434 7/1960 Eichler et a1. .....96/l A UK 3,532,494 10/ 1 970 Bhagat ..96/1 3,449,568 6/1969 Vock ..96/1 X Primary Examiner-George F. Lesmes Assistant Examiner-John R. Miller Attorney-James J. Ralabate, David C. Petre and Michael H.

Shanahan [s7 ABSTRACT Build-up of undesired potential on the blocking electrode in a photoelectrophoretic imaging system during successive imaging operations is prevented by one of several methods. Typically, potential of a sign opposite to that built up may be applied, or the blocking electrode surface may be discharged between imaging cycles.

8 Clains, 4 Drawing Figures INVENTOR, ROGER N. CICCARELL P'ATENTEDJuL 1 1 I972 3. 676 3 1 3 sum 20F 2 FIG. 2

FIG. 4

REMOVING UNDESIRED POTENTIAL FROM THE BLOCKING ELECTRODE IN A PHOTOELECTROPHORETIC IMAGING SYSTEM CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuing application of my copending application Ser. No. 626,917, filed Mar. 28, 1967, now abandoned.

BACKGROUND OF THE INVENTION This invention relates in general to imaging systems. More specifically, the invention concerns an improved photoelectrophoretic imaging system.

There has been recently developed an electrophoretic imaging system capable of producing color images which utilizes electrically photosensitive particles. This process is described in detail and claimed in copending applications, Ser. Nos. 384,737, now U.S. Pat. No. 3,384,565; 384,680 and 384,681, both now abandoned all filed July 23, i964. In such an imaging system, variously colored light absorbing particles are suspended in a non-conducting liquid carrier. The suspension is placed between electrodes, one of which is generally conductive called the injecting electrode and the other of which is generally insulating and called the blocking electrode. One of these electrodes is at least partially transparent. The suspension is subjected to a potential difference between the electrodes across the suspension and exposed to an image through said partially transparent electrode. As these steps are completed selective particle migration takes place in image configuration, providing a visible image at one or both of the electrodes. An essential component of the system is the suspended particles which must be electrically photosensitive and which apparently undergo a net change in charge polarity upon exposure to activating electromagnetic radiation, through interaction with one of the electrodes. In a monochromatic system, particles of a single color are used, producing a single colored image equivalent to conventional black-andwhite photography. ln a polychromatic system, the images are produced in natural color because mixtures of particles of two or more different colors which are each sensitive to light of a specific wavelength or narrow range of wavelengths are used.

This system, using a conductive injecting electrode, a substantially insulating blocking electrode and photosensitive particles therebetween in an insulating carrier liquid has been found to be capable of producing excellent images. It has been found that when the blocking electrode surface is highly insulating, e.g., has a resistivity of ohm-centimeter or greater, the highest image quality is obtained. On the other hand, image quality is generally lower where the resistivity of the blocking electrode surface material is 10 ohm-centimeters or less. Also, where several images were made using the same electrodes in rapid succession image quality has been found to fall off greatly. While a great many substantially insulating materials are operative when used as the surface for the blocking electrode, i.e., will produce satisfactory images, very few materials have been found which consistently produce high quality images.

Many of the physical properties of those materials which are capable of producing excellent images are not ideal. For example, Baryta paper surfaced blocking electrodes produce excellent images, however, the Baryta paper surface is humidity sensitive and is not easily cleaned between imaging steps. The limited number of materials known to be useful do not have as wide a range of humidity resistance, tractability, cleanability, surface smoothness, abrasion resistance and cost as would be desirable. Thus, there is a continuing need for improved blocking electrode materials and for methods of using the blocking electrode which would permit utilization of a wider range of materials on the blocking electrode surface.

lt is, therefore, an object of this invention to provide an electrophoretic imaging system overcoming the above noted disadvantages.

It is another object of this invention to provide an electrophoretic imaging system capable of utilizing a wide range of blocking electrode materials.

It is another object of this invention to provide a photoelectrophoretic imaging system capable of producing a plurality of images in rapid succession of uniformly high quality.

It is another object of this invention to permit the use of all highly insulating materials in the blocking electrode of photoelectrophoretic imaging systems.

The above objects, and others, are accomplished by providing a photoelectrophoretic imaging system in which the buildup of undesired charge on the blocking electrode is eliminated. lt has been discovered that prior blocking electrodes having surfaces of highly insulating materials performed in a less than optimum manner due to the build-up of undesired electrostatic charge on the surface. This problem is especially great where several images are made in rapid succession since undesired charge continues to build-up on the surface of the blocking electrode causing steady degradation of successive images. The undesired charge includes that charge which tends to reduce the potential difference and therefore the electric field strength between the electrodes.

As further pointed out below, the undesired charge build-up on a surface of the blocking electrode may be eliminated or neutralized in several different ways. Typically, the surface of the blocking electrode may be brushed with a conductive brush between successive imaging cycles or the electrode surface may be treated with an opposite polarity potential sufficient to neutralize the surface charge between successive imaging cycles. Any suitable method may be used to apply a neutralizing charge to the blocking electrode surface to eliminate undesired charge thereon. Typical methods include AC or DC corona discharge as taught by Carlson in U.S. Pat. No. 2,588,699 and Walkup in U.S. Pat. No. 2,777,957; triboelectric charging as taught by Carlson in U.S. Pat. No. 2,297,691; charge transfer from a conductive roller as taught by Tregay et al. in U.S. Pat. No. 2,980,834; charge transfer from a conductive liquid as taught by Walkup in U.S. Pat. No. 2,987,660 and any desirable combination of these methods.

The various features, advantages and objects of the present invention will become further apparent from the following description and drawings in which:

FIG. 1 shows a schematic representation of a single electrophoretic imaging system;

FIG. 2 shows a blocking electrode immediately after image formation;

FIG. 3 shows an embodiment in which the blocking electrode surface is grounded between imaging steps; and

FIG. 4 shows an alternative embodiment for eliminating charge build-up on the blocking electrode.

Referring now to FIG. I, there is seen a transparent electrode generally designated 1, which in this exemplary instance, is made up of a layer of optically transparent glass 2 overcoated with a thin optically transparent layer 3 of tin oxide, commercially available under the name NESA glass. This electrode will hereafter be referred to as the injecting" electrode. Coated on the surface of injecting electrode 1 is a thin layer 4 of finely divided photosensitive particles dispersed in an insulating liquid carrier. Photosensitive," for the purposes of this application, refers to the properties of a particle which, once attracted to the injecting electrode, will migrate away from it under the influence of an applied electric field when it is exposed to actinic electromagnetic radiation. For a detailed theoretical explanation of the apparent mechanism of operation of the imaging process, see the above mentioned copending applications, Ser. Nos. 384,737; 384,681, and 384,680, the disclosures of which are incorporated herein by reference. Adjacent to the liquid suspension 4 is a second electrode 5, hereinafter called the blocking electrode" which is connected to one side of the potential source 6 through a switch 7. The opposite side of potential source 6 is connected to the injecting electrode 1 so that when switch 7 is closed, an electric field is applied across the liquid suspension 4 between electrodes l and 5. An image projector made up of a light source 8, a transparency 9, and a lens is provided to expose the dispersion 4 to a light image of the original transparency 9 to be reproduced. Electrode 5 is made in the form of a roller having a conductive central core 11 connected to the potential source 6. The core is covered with a layer of a blocking electrode material 12, which may be any suitable insulating material as further discussed below. The particle suspension is exposed to the image to be reproduced while a potential is applied across blocking and injecting electrodes by closing switch 7. Roller 5 is caused to roll across the top surface of injecting electrode 1 with switch 7 closed during the period of image exposure. This light exposure causes exposed pigment particles originally attracted to electrode 1 to migrate through the liquid and adhere to the surface of transferred blocking electrode, leaving behind a particulate image on the injecting electrode surdace which is a duplicate of the original transparency 9. After exposure, the relatively volatile carrier liquid evaporates off, leaving behind the particulate image. This particulate image may then be fixed in place, as for example, by placing a lamination over its top surface or by virtue of a dissolved binder material in the carrier liquid such as paraffin wax or other suitable binder that comes out of the solution as the carrier liquid evaporates. In an alternative, the particulate image remaining on the injecting electrode may be transferring to another surface and fixed thereon. This system can produce either monochromatic or polychromatic images depending upon the type and number of pigments suspended in the carrier liquid and the color of light to which this suspension is exposed in the process.

Where the above imaging steps are repeated, with cleaning of the blocking electrode and reapplication of the particle suspension onto the injecting electrode between imaging operations, it has been found that there is a steady decrease in image quality in successive copies. It has now been found that this gradual decrease in image quality is due to the build-up of undesired electrostatic charge on the surface of the blocking electrode. FIG. 2 shows the blocking electrode after imaging where the blocking electrode surface 12 consists of a highly insulating material. In this instance, a negative potential was maintained on the blocking electrode core 11 during imaging. FIG. 2 shows the blocking electrode immediately after imaging. As the roller moved across the injecting electrode an apparent positive charge was built up on the surface of blocking electrode material 12. Thus, even after the switch 7 is opened, a potential remains across the blocking electrode material 12. Where the blocking electrode material is at least slightly conductive, charge can gradually migrate through the blocking electrode material 12 neutralizing the surface charge. Once the charge has thus been neutralized an acceptable image may again be produced. However, in rapid automatic equipment, there is often insufficient time between imaging operations to permit the surface charge to leak away. Also, most materials having desirable physical properties for use as the surface of a blocking electrode are not sufficiently conductive to allow the built-up charge to leak off within a reasonable time.

One embodiment of a system for removing undesired surface charge from a blocking electrode is shown in FIG. 3. In this schematic representation, immediately after completion of an imaging operation, a conductive brush 13 is brought into contact with the blocking electrode surface and the blocking electrode roller is rotated thereagainst. If desired, brush 13 may perform the dual functions of removing undesired surface charge and may also be used to clean unwanted particles from the blocking electrode surface thus readying the blocking electrode roller for successive imaging operations. To enable the conductive brush 13 to remove the undesired surface charge it is preferred to have the brush l3 maintained at a potential level that can compete with the voltage level coupled to the core of the blocking electrode roller. Consequently, the brush 13 should be coupled to substantially the same or a more negative voltage (more positive if opposite polarities are being used) than the voltage coupled to the blocking electrode. The shorting wire 20 coupled to brush l3 and to the core of roller 5 maintains the brush at substantially the same voltage level as the blocking electrode 5. As roller 5 and conductive brush l3 rotate against each other, particles from the imaging suspension 4 which have migrated to the blocking electrode 5 will be cleaned from the surface of blocking electrode material 12. If desired, the conductive brush may be maintained at any suitable potential so as to leave a slight surface charge on the blocking electrode roller after cleaning. A suitable potential may be a potential that increases rather than decreases the electric field between the electrodes.

FIG. 4 shows an alternative embodiment of the means for eliminating surface charge on the blocking electrode surface after imaging. As shown in this Figure, the surface charge remaining on the blocking electrode surface 12 may be neutralized after imaging by rotating the roller 5 adjacent a DC corona discharge unit 14 with power supply 15 held at a polarity opposite to that of the undesired charge on the blocking electrode surface 12. Alternatively, an AC corona unit may be used which will reduce the potential on the blocking electrode to substantially zero potential. Any suitable corona discharge unit may be used, such as those described in US. Pat. No. 2,588,699 to Carlson and US. Pat. No. 2,777,957 to Walkup. The surface charge on blocking electrode 12 may be, alternatively, neutralized by charge applied by other conventional methods. For example, a charge of a potential opposite to the undesired surface charge may be applied triboelectrically by rubbing the blocking electrode surface with suitable material as taught by Carlson in US. Pat, No. 2,297,691. Also, the opposite potential may be applied by contacting the blocking electrode surface with a conductive roller or conductive liquid held at the desired potential as taught by Tregay et al. in US. Pat. No 2,980,834 and Walkup in U.S. Pat. No. 2,987,660.

The roller blocking electrode configuration shown in the drawings is, of course merely representative and any other suitable configuration may be used. Typically, the blocking electrode could be in the form of a movable or stationary flat plate, or in the form of a belt entrained over rollers. The blocking electrode surface may comprise any suitable material having a volume resistivity of at least 10 ohm-centimeters. Where the resistivity of the blocking electrode material is lower than 10 ohm-centimeters there is a tendency for unwanted pigments which should migrate to the blocking electrode surface and adhere thereto to be reflected back towards the injecting electrode thereby degrading image quality. For highest image quality, it is preferred that the blocking electrode have a volume resistivity of at least 10 ohm-centimeters. In the range of 10 to 10 ohm-centimeters the surface charge which develops on the surface of the blocking electrode during imaging can be eliminated merely by a suitable delay between imaging operations to permit surface charge to leak off. However, it is preferred that a means be provided for eliminating the undesired surface charge since in high speed automatic imaging equipment it is desirable to produce successive images rapidly without the delay necessary to allow the surface charge to leak off. Where the volume resistivity of the blocking electrode material is greater than 10 ohm-centimeters the time necessary for dissipation of the surface charge is great enough to require external means for removing said charge. Also, most of the polymeric materials having desirable physical properties have volume resistivities greater than 10 ohm-centimeters. Thus, one may select from a very great number of possible blocking materials when external means is provided to remove unwanted charge after imaging. As stated above, any suitable material having the desired resistivity may be used. Typical materials include vinyl polymers; polyolefins such as polyethylene, polypropylene, polyisobutylene; polyaromatics such as polystyrene, polyalkyds, polyvinyl toluene, polyphenylene oxides, polysulfone, polyxylylenes; polyacrylics and their esters; polyhalocarbons such as vinyl and vinylidene chlorides and fluorides; polyperfluon'nated hydrocarbons such as polytetrafluoroethylene; polyvinyl esters; polyvinyl acetates; polyvinyl acetals and ketals such as polyvinyl butyral; phenolic resins; polyesters; polyethers; silicone resins; polycarbonates; epoxy resins; polyamides; polyimide; urethane resins; polysulfides and copolymers and mixtures thereof.

The blocking electrode material may include dopants or additives to modify resistivity or other physical properties for particular uses. For example, resistivity may be modified by the addition of carbon black, conductive pigments and dyes, powdered metals, inorganic salts, etc.

Any suitable insulating liquid may be used as the carrier for the pigment particles in the system. Typical carrier liquids include decane, dodecane, N-tetradecane, paraffin, beeswax or other thermoplastic materials, Sohio Odorless Solvent 3440 (a kerosene fraction available from the Standard Oil Company of Ohio), lsopar-G (a long chain saturated aliphatic hydrocarbon available from Humble Oil Company ofNew Jersey) and mixtures thereof. Good quality images have been produced with voltages ranging from about 300 to about 5,000 volts with either a negative or positive polarity on the blocking electrode core.

Any suitable photosensitive particle or mixtures or such particles may be used in carrying out the imaging process, regardless of whether the particular particle selected is organic, inorganic and is made up of one or more components in solid solution or dispersed one in the other or whether the particles are made up of multiple layers of different materials. Typical photosensitive particles include organics such as 8,13-dioxodinaphtho-(1,2,2,3)-furan-6-carbox-p-methoxyanilide; Locarno Red, C.I. No. 15865, l-(4-methyl-5'- chloroazobenzene-2-sulfonic acid)-2-hydroxy-3-naphthoic acid; Watchung Red B, the barium salt of l-(4'-methyl-5- chloroazobenzene-2'-sulfonic acid)-2-hydroxy-3-naphthoic acid, C.I. No. 15865; Naphthol Red B, l-(2'-methoxy-5'- nitrophenylazo)-2-hydroxy-3 "-nitro-3-naphthanilide, C.I. No. 12355; Duol Carmine, the calcium lake of 1-(4- methylazobenzene-2'-sulfonic acid)-2-hydroxy-3-naphthoic acid, C.I. No. 15850; Calcium Lithol Red, the calcium lake of 1-(2'-azonaphthalene-1'-su1fonic acid)-2-naphthol, C.I. No. 15630; Quinacridone and substituted quinacridones such as 2,9-dimethylquinacridone; Pyranthrones; Indofast Brilliant Scarlet Toner, 3,4,9,10-bis (N,N-(p-methoxyphenyl-imido)- perylene, C.I. No. 7 l 140; dichloro thioindigo; Pyrazolone Red B Toner, C.I. No. 21120; phthalocyanines including substituted and unsubstituted metal and metal-free phthalocyanines such as copper phthalocyanine, magnesium phthalocyanine, metal-free phthalocyanine, polychloro substituted phthalocyanines etc; Methyl Violet, a phosphotungstomolybdic acid lake of a Triphenylmethane dye, C.I. No. 42535; Indofast Violet Lake, dichloro-9,l8-isoviolanthrone, C.I. No. 60010; Diane 'Blue, 3,3 '-methoxy-4,4-diphenyl-bis( l "-azo- 2"-hydroxy-3-naphthanilide, C.I. No. 21180; lndanthrene Brilliant Orange R.K., 4,l0-dibromo-6,lZ-anthanthrone, C.I. No. 59300; Algol Yellow G.C.; 1,2,5,6-di(C,C'-diphenyl)- thiazole-anthraquinone, C. 1. No. 67300; Flavanthrone; Indofast Orange Toner, C.I. No. 71 105; l-cyano-2,3-phthaloyl- 7, 8-benzopyrrocoline and many other thio indigos, acetoacetic arylides, anthraquinones, perionones, perylenes, dioxazines, quinacridones, azos, diazos, thoazines, azines and the like; inorganics such as cadmium sulfide, cadmium sulfoselenide, zinc oxide, zinc sulfide, sulphur selenium, mecuric sulfide, lead oxide, lead sulfide, cadmium selenide titanium dioxide, indium trioxide, and the like. In addition to the aforementioned pigments other organic materials which may be employed in the particles include polyvinylcarbozole; 2,4-bis (4,4'-diethyl-aminophenyl)-1,3,4-oxidiazole; N-isopropylcarbozole; polyvinylanthracene; triphenylpyrrol; 4,5-diphenylimidazolidinone; 4,5-diphenylimidazolidinone; 4,5-diphenylimidazolidinethione; 4,5-bis-(4'-amino-phenyl)- imidazolidinone; l,2,5,6-tetraazacyclooctatetraene-(2,4,6,8); 3.4-di-(4'-phenoxy-phenyl)-7,8-diphenyl-I ,2,5 ,6-tetraazacyclooctatetraene-(2,4,6,8); 3,4-di(4'-phenoxy-phenyl)-7,8- diphenyl-l ,2,5,6-tetraaza-cyclooctatetraene-(2,4,6,8);

3 ,4,7 ,8-tetramethoxyl ,2,5 ,-tetraazacyclooctatetraene- (2,4,6,8); 2-mercapto-benzthiazole; 2-phenyl-4-alphanuphthylidene-oxazolone; 2-phenyl-4-diphenylideneoxazolone; 2-phenyl-4-p-methoxybenzylideneoxazolone; 6- hydroxy-2-phenyl(p-dimethyl-amino phenyl)-benzofurane; 6-

hydroxy-2,3-di(p-methoxyphenyl)-benzofurane; 2,3,5,6- tetra(p-methoxyphenyl)-furo(3,2f)-benzofurane; 4- dimethylaminobenzylidene-benzhydrazide; 4-dimethylaminobenzylideneisonicotinic acid hydrazide; turfurylidene- (2)-4-dimethylamino-benzhydrazide; 5-benzilidene-aminoacenaphthene-3-benzylidene-amino-carbazole; (4-N,N- dimethylamino-benzylidene)-p-N,N-dimethylaminoaniline; (2-nitro-benzylidene)-p-bromo-aniline; N,N-dimethyl-N'-(2 nitro-4-cyanobenzylidene)-p-phenylenediamine; 2,4-diphenyl-quinazoline; 2-(4-amino-phenyl)-4-phenyl quinazoline; 2- phenyl-4-(4 '-di-methyl-amino-phenyl )-7-methoxy-quinazoline; l,3-diphenyl-tetrahydroimidazole; 1,3-di-(4 chlorophenyl)-tetra-hydroimidazole; l ,3diphenyl-2 ,4 dimethyl aminophenyl)-tetra-hydroimidazole; 1,3-di-(ptolyl)-2-[quinolyl- 2) ]-tetrahydroimidazole; 3-(4-dimethylamino-phenyl)-5-(4"-methoxy-phenyl)-6-phenyl- 1,2,4,-triazine; 3-pyridil-(4')5-(4"-dimethylaminophenyl-6- phenyl-l ,2,4-triazine; 3(4 '-amino-phenyl )-5,6-di-phenyl- 1,2,4-triazine; 2,5-bis [4-amino-phenyl-(1) ]-l,3,3,-triazole; 2,5-bis [4-(N-ethyl-N-acetyl-amino)-phenyl-(4')]-1,3,4- triazole; 1,5-diphenyl-3-methyl-pyrazoline; 1,3,4,5-tetraphenyl-pyrazoline; l-phenyl-3-(p-methoxy styrl)-5-(p-methoxyphenyl)-pyrazoline; l-methyl-2(3 ,4'-dihydroxy-methylenephenyl)-benzimidazole; 2-(4-dimethylamino phenyl)-benzoxazole; 2-(4'-methoxyphenyl)-benzthiazole; 2,5-bis [p-aminophenyl-( 1) ]-1,3,4-oxidiazole; 4,5-diphenyl-imidazolone; 3- amino-carbazole; copolymers and mixtures thereof.

Other materials include organic donor-acceptor (Lewis acid-Lewis base) charge transfer complexes made up of donors such as phenolaldehyde resins, phenoxides, epoxies, polycarbonates, urethanes, styrene or the like complexed with electron acceptors such as 2,4,7-trinitro-9-fluorenone; 2,4,5,7-tetranitro-9-fluorenone; picric acid; 1,3,5-trinitro benzene; chloranil; 2,5-dichloro-benzoquinone; anthraquinone-2-carboxylic acid, Bromal, 4-nitro-phenol; maleic anhydride; metal halides of the metals and metalloids of groups 1-3 and lI-Vlll of the periodic table including, for example, aluminum chloride, zinc chloride, ferric chloride, magnesium chloride, calcium iodide, strontium bromide, chromic bromide, arsenic triiodide, magnesium bromide, stannous chloride etc.; boron halides, such as boron trifluorides; ketones such as benzophenone and anisil, mineral acids such as sulfuric acid; organic carboxylic acids such as acetic acid and maleic acid, succinic acid, citroconic acid, sulphonic acid, such as 4-toluene sulphonic acid and mixtures thereof. In addition to the charge transfer complexes it is to be noted that many additional ones of the above materials may be further sensitized by the charge transfer complexing technique and that many of these materials may be dye-sensitized to narrow, broaden or heighten their spectral response curves.

As stated above, any suitable particle structure may be employed. Typical particles include those which are made up of only the pure photosensitive material or a sensitized form thereof, solid solutions or dispersions of the photosensitive material in a matrix such as thermoplastic or thermosetting resins, copolymers of photosensitive pigments and organic monomers, multilayers of particles in which the photosensitive material is included in one of the layers and where other layers provide light filtering action in an outer layer or a fusable or solvent softenable core of resin or a core of liquid such as dye or other marking material or a core of one photosensitive material coated with an overlayer of another photosensitive material to achieve broadened spectral response. Other photosensitive structures include solutions, dispersions, or copolymers of one photosensitive material in another with or without other photosensitivity inert materials. Other particle structures which may be used but which are not required include those described in US. Pat. No. 2,940,847 to Kaprelian.

Although various electrode spacings may be employed, spacings of less than 1 mil and extending down even to the point where the electrodes are pressed together as in the case of the roller electrode constitute a particularly preferred form of the invention in that they produce better resolution and superior color separation results than is produced with wider spacings. This improvement is believed to take place because of the high field strength across the suspension during imagmg.

In a monochromatic system, particles of a single color are dispersed in the carrier liquid and exposed to a black-andwhite image. A single color image results, corresponding to black-and-white photography. In a polychromatic system, the particles are selected so that those of different colors respond to different wavelengths in the visible spectrum corresponding to their principal absorption bands. Also, the pigments should be selected so that their spectral response curves do not have substantial overlap, thus allowing for color separation and subtractive multi-color image formation. In a typical subtractive multi-color system, the particle dispersion should include cyan colored particles sensitive mainly to red light, magenta particles sensitive mainly to green light and yellow particles sensitive mainly to blue light. When mixed together in a carrier liquid, these particles produce a black appearing liquid. When one or more of the particles are caused to migrate from the injecting electrode towards the blocking electrode, they leave behind particles which produce a color equivalent to the color of the impinging light. Thus, for example, red light exposure causes the cyan colored particles to migrate, leaving behind the magenta and yellow particles which combine to produce red in the final image. In the same manner, blue and green colors are reproduced by the removal of yellow and magenta respectively. When white light impinges upon the mix, all particles migrate, leaving behind the color of the white or transparent substrate. No exposure leaves behind all pigments which combine to produce a black image. This is an ideal technique of subtractive color imaging in that the particles are not only each composed of a single component but, in addition, they perform the dual functions of final image colorant and photosensitive medium, Typical photosensitive pigments include those described in copending applications Ser. No. 473,607, filed July 21, 1965; Ser. No. 421,281, filed Dec. 28, 1964, now U.S. Pat. No. 3,447,922 and Ser. No. 445,240, filed Apr. 2, 1965, now U.S. Pat. No. 3,384,632.

The following examples further specifically define the present invention with respect to the elimination of undesired surface charge build-up on the blocking electrode in photoelectrophoretic imaging systems. Parts and percentages are by weight unless otherwise indicated. The examples below are intended to illustrate various preferred embodiments of the photoelectrophoretic imaging system of the present invention.

All the following examples are carried out in an apparatus of the general type illustrated in FIG. 1 with the addition in certain examples of surface charge removing means such as those shown in FIGS. 3 and 4. The imaging mix comprising the desired photosensitive particles in an insulating carrier liquid is coated on a NESA glass substrate through which exposure is made. The NESA glass surface is connected in series with a switch, a potential source, and the conductive center of a roller having a coating of blocking electrode material on its surface. The roller is approximately 2% inches in diameter and is moved across the plate surface at about 4 centimeters per second. The plate employed is roughly three inches square and is exposed to the light intensity of about 2,000 foot candles as measured on the uncoated NESA glass surface. All pigments which have a relatively large particle size as received commercial or is made are ground in a ball mill for about 48 hours to reduce their size in order to provide a more stable dispersion and improve the resolution of the final images. In all examples, measurements of resistivity of insulating materials are made according to ASTM D257-6l methods.

EXAMPLEI A blocking electrode in roller configuration is prepared consisting of a metal core surrounded by carbon black filled rubber having a resistivity of 10 ohm-centimeters and having on the surface a 2 mil Lexan (a polycarbonate available from General Electric Company) film as the blocking electrode surface resistivity of about 10 ohm-cm. A tri-mix is prepared consisting of about 7 parts photosensitive pigment dispersed in about parts Sohio Odorless Solvent 3440. The photosensitive pigments comprise equal portions of a cyan pigment, Monolite Fast Blue GS, the alpha form of metal-free phthalocyanine available from the Arnold Hoffman Company; a magenta pigment, Watchung Red B, 1-(4'-methyl-5'- chloroazobenzene-Z-sulfonic acid)-2-hydroxy-3-naphthoic acid, available from E. I. duPont de Nemours & Company and a yellow pigment, N-2"-pyridyl-8, l 3-dioxodinaphtho-( 1 ,2-2 '3 )-furan-6-carboxamide, prepared by the method disclosed in copending application Ser. No. 421,281, filed Dec. 28, 1964. This dispersion is coated onto the NESA injecting electrode to a thickness of about 3 mils, a negative potential of about 3,000 volts is imposed on the core of the blocking electrode roller and the roller is rolled across the injecting electrode surface while the suspension is exposed to an image from a conventional Kodachrome transparency. When the roller has passed beyond the injecting electrode, potential application and image exposure are stopped. The blocking electrode surface is then cleaned manually using Kimwipes 900-L (a tissue type disposable wiper available from Kimberly Clark) moistened with Sohio Odorless Solvent 3440. The image produced on the injecting electrode is observed to be of excellent quality and good color balance. The image is transferred to a receiving sheet by the method described in copending application Ser. No. 542,050, filed Apr. 2, 1966, now U.S. Pat. No. 3,565,614. The roller electrode is then returned to the starting position and the coating, imaging, cleaning and transfer steps are repeated seven additional times. The average time between imaging steps is about 1 hour. The images produced by the successive imaging steps are then compared. It is observed that there is a gradual loss in color density, primarily in blue areas, in succeeding images. A shift towards magenta is observed and an overall drastic loss in density is observed in the last three images produced.

TheLexan film blocking electrode surface is then replaced with a fresh piece of Lexan film. Eight images are successively produced as described above, except in this instance after each cleaning step the blocking electrode is rotated slowly for about 30 seconds in contact with a metal brush coupled to the same potential as the blocking electrode of the sort shown in FIG. 3. The eight images produced are then compared. All eight images are of substantially uniform high quality with excellent color balance.

EXAMPLE II A blocking electrode and a pigment dispersion are prepared as in Example I. The imaging operations of Example 1 are repeated except that after each cleaning step the blocking electrode is rotated slowly for about 1 minute while an AC corona discharge unit, available from Herbert Products, Inc. Westbury, New York, under the tradename Curastat Static Eliminator SE104 is operated at about one-eighth inch from the blocking electrode surface. The blocking electrode surface is electrometered with a Monroe Electrostatic Voltmeter (Monroe Electronic Laboratories, Inc., Middleport, New York) after the AC corona treatment and found to have substantially zero charge remaining thereon. Each of the eight images produced with the AC corona treatment between imaging steps is of substantially equal high color quality.

EXAMPLE III A blocking electrode and a pigment tri-mix are prepared as in Example I. The imaging tests of Example I are repeated except that after each cleaning step, the blocking electrode is rotated slowly for about 1 minute adjacent a DC corona discharge unit. The DC discharge wire is held about onefourth inch from the blocking electrode surface and is maintained at a negative potential of about 5,000 volts. The surface of the blocking electrode is continuously electrometered and DC corona discharge is stopped when the charge on the blocking electrode surface is substantially eliminated or is slightly negative. The eight images produced by successive imaging operations utilizing the DC corona discharge after each image is formed are then compared. They are found to be of uniform high color quality and excellent color balance.

EXAMPLE IV The experiments of Examples l and II are repeated using a blocking electrode which consists of a metal core having a diameter of about 2 V1 inches which has on the surface thereof a 2 mil Mylar (polyethylene terephthalate available from E.I. duPont de Nemours & Company) film. The eight images produced as in Example I, with no treatment of the blocking electrode surface other than cleaning between image operations show a gradual definite decrease in image quality with successive images. A loss in density in blue image areas is immediately noticeable. With further images an overall density loss and shift towards the magenta is observed. In the eight images produced as in Example I, with charge elimination from the blocking electrode surface after each cleaning step by means of the conductive brush, the images produced are of substantially uniform high quality with consistently good color balance and image density. The eight images produced as in Example II, with discharge of the blocking electrode surface by AC corona discharge between imaging operations, are also of uniform high quality.

EXAMPLE V A blocking electrode in roller configuration is prepared consisting of a metal core surrounded by carbon black filled rubber having a volume resistivity of about ohm-centimeters and having on the surface a 2 mil Tedlar (a polyvinyl fluoride material from El. duPont de Nemours & Company) film as the blocking electrode surface. A tri-mix is prepared as in Example I and coated onto the NESA injecting electrode to a thickness of about 3 mils. A negative potential of about 3,000 volts is imposed on the core of the blocking electrode roller and the roller is rolled across the injecting electrode surface while the suspension is exposed to an image from a conventional Kodachrome transparency. When the blocking electrode roller has passed beyond the injecting electrode surface, potential application and image exposure are stopped. The blocking electrode surface is then cleaned. The image produced on the injecting electrode is observed to be of excellent quality and good color balance. The image is transferred to a receiving sheet and the coating, imaging, cleaning and transfer steps are repeated seven additional times. The average time between the imaging steps is about 20 seconds. The images produced by the successive imaging steps are then compared. A gradual loss in color density is observed with succeeding images. The Tedlar film blocking electrode surface is then replaced with a fresh piece of Tedlar film. Eight images are successively produced as described above, except in this instance, there is a delay of about 10 minutes between imaging operations. The eight images produced are then compared. All images are of good quality. Since Tedlar has a volume resistivity of about 10 ohm-cm., it is apparent that substantially all of the charge built-up on the blocking electrode surface during the imaging operation is able to leak away during the 10 minute delay between the imaging operations.

EXAMPLE VI The experiments of Examples l and II are repeated except that in this instance a monochromatic imaging suspension is used and the image is exposed to a black-and-white image. The uni-mix consists of about 7 parts 2,4,6-tris(N-ethyl-N- hydroxyethyl-p-amino-phenylazo) phluoroglucinol prepared as described in copending application Ser. No. 473,607, filed July 21, 1965, dispersed in about parts lsopar-G. This suspension is then imaged as described in Examples 1 and ll. With no treatment to eliminate charge build-up between imaging steps, image quality steadily decreases with succeeding images. Principally, a drastic fall off in image density is observed. Where the build-up of undesired charge is eliminated either by use of the conductive brush as in Example I or by the corona discharge as in Examples II and III, the images produced are of consistently high quality with little or no loss of image density in succeeding images.

EXAMPLE Vll The experiments of Examples l and ll are repeated except that here a different tri-mix is used. A mixture of photosensitive pigments is prepared by mixing equal parts of a cyan pigment, Methyl Violet, C.l. No. 42535, a phosphotungstomolybdic acid lake of 4(N,N',N-trimethyl anilino)-methylene-N", N"-dimethyl anilinium chloride, available from Collway Colors; magenta pigment, Naphtho Red B, C.l. No. 12355, I- (2-methoxy-5'-nitrophenylaz0)-2-hydroxy-3-nitro-3- naphthanilide, available from Collway Colors; and a yellow pigment, Algol Yellow GC, C]. No. 67300, l,2,5,6-di(C,C'- diphenyl)-thiazoleanthraquinone, available from General Dyestuffs. About 8 parts of this pigment mixture is suspended in about 100 parts of mineral oil. Twenty-four images are formed using this suspension as described in Examples 1 and ll. Where no steps are taken to prevent build-up of unwanted charge during the imaging operations, there is a steady decrease in image quality with succeeding images. After four images are made there is a drastic decrease in color density. Where build-up of unwanted charge on the blocking electrode surface is eliminated by contact with a grounded conductive brush as in Example I and by exposure to AC. or DC. corona as in Examples II and III the images produced have consistently high quality with no discernible loss in color density.

EXAMPLE Vlll A blocking electrode in roller configuration is prepared as in Example 1, except that the surface layer consists of Teflon FEP (a fiuorinated ethylene-propylene copolymer available from E. I. duPont de Nemours & Company), film having a thickness of about 3 mils. A tri-mix is prepared as in Example I, but is coated to a thickness of about 3 mils onto the blocking electrode surface instead of onto the NESA electrode. Twenty-four images are produced, using this configuration, as described in Examples I and II. Where there is no treatment of the blocking electrode to eliminate charge build-up there is a gradual, definite loss of image quality with succeeding images. Where charge build-up is eliminated by grounding an A.C. corona treatment of the blocking electrode between imaging operations, image quality remains high through each set of 8 succeeding images.

Although specific components and proportions have been described in the above examples relating to methods of electrophoretic imaging, other suitable materials and process steps, as listed above, may be used with similar results. In addition, other materials may be added to the electrodes, photosensitive particles, or particulate suspensions to synergize, enhance, or otherwise modify their properties. For example, the photosensitive particles may be dye sensitized or electrically sensitized if desired, or be mixed with any other photosensitive materials both organic and inorganic.

Other modifications and ramifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of this invention.

What is claimed is:

1. A photoelectrophoretic imaging process for sequentially producing a plurality of images comprising providing an injecting electrode and a blocking electrode at least one of which is transparent,

introducing a suspension of photosensitive particles between said electrodes for the production of each of said plurality of images,

imposing an electric field across the suspension between said electrodes for the production of each of said plurality of images,

exposing the suspension between said electrodes through the transparent electrode to imagewise activating electromagnetic radiation for the production of each of said plurality of images,

separating said electrodes during the production of each of said plurality of images after the exposure to radiation and application of field to obtain an image on one of said electrodes and wherein undesirable charge capable of reducing the field imposed across a suspension appears on the blocking electrode, and

eliminating said undesirable charge from said blocking electrode between the production of images while the electrodes are separated whereby the image quality of the sequentially produced plurality of images is substantially uniform.

2. The process of claim 1 wherein said charge is eliminated by contacting the surface of the blocking electrode with a conductive member having a voltage potential coupled to it at least substantially the same level as a voltage potential coupled to the blocking electrode.

3. The process of claim 1 wherein said undesirable charge is eliminated by exposing the surface of said blocking electrode to DC. corona discharge of polarity opposite to said charge.

4. The process of claim 1 wherein said undesirable charge is eliminated by exposing the surface of said blocking electrode to AC. corona discharge.

5. The process of claim 1 wherein said photosensitive particles comprise cyan colored particles primarily responsive to red light, magenta colored particles primarily sensitive to green light and yellow colored particles primarily responsive to blue light and said image formed is a subtractive polychromatic images.

6. The process of claim 2 wherein said conductive member is a conductive rotary brush.

7. The process of claim 1 wherein said blocking electrode has a resistivity of from about l0 ohm-centimeters or greater.

8. The process of claim I wherein said blocking electrode has a resistivity of from about 10 to l0 ohm-centimeters. 

2. The process of claim 1 wherein said charge is eliminated by contacting the surface of the blocking electrode with a conductive member having a voltage potential coupled to it at least substantially the same level as a voltage potential coupled to the blocking electrode.
 3. The process of claim 1 wherein said undesirable charge is eliminated by exposing the surface of said blocking electrode to D.C. corona discharge of polarity opposite to said charge.
 4. The process of claim 1 wherein said undesirable charge is eliminated by exposing the surface of said blocking electrode to A.C. corona discharge.
 5. The process of claim 1 wherein said photosensitive particles comprise cyan colored particles primarily responsive to red light, magenta colored particles primarily sensitive to green light and yellow colored particles primarily responsive to blue light and said image formed is a subtractive polychromatic images.
 6. The process of claim 2 wherein said conductive member is a conductive rotary brush.
 7. The process of claim 1 wherein said blocking electrode has a resistivity of from about 10 13ohm-centimeters or greater.
 8. The process of claim 1 wherein said blocking electrode has a resistivity of from about 10 7 to 10 13 ohm-centimeters. 